Fluid intelligence is tied to biology. It is defined as our "on-the-spot reasoning ability, a skill not basically dependant on our experience." (Belsky, 1990, p. 125) Belsky (1990) indicates this type of intelligence is active when the central nervous system (CNS) is at its physiological peak.

Fluid intelligence is measured by the performance subtasks on the Wechsler Adult Intelligence Scale (WAIS).

Fluid intelligence is important to psychologists as it relates to the study of aging. There is ongoing intense debate among psychologists as to whether or not intelligence declines with aging. Belsky (1990) claims fluid intelligence "reaches a peak in early adulthood and then regularly declines." (p. 125) This is because of the physiological changes that accompany aging. "The development of CNS structures is exceeded by the rate of CNS breakdown." (Horn, 1970 as quoted in Belsky, 1990, p. 125)